1. Field of the Invention
This invention relates to a driving circuit for a vibration wave motor.
2. Related Background Art
A vibration wave motor for frictionally driving a moving member by the utilization of a travelling vibration wave is recently being put into practical use. The principle of operations of such motor will hereinafter be described with reference to FIG. 3 of the accompanying drawings.
A ring-like vibration member 3 of resilient material whose full peripheral length is an integral number of as great as a certain length .lambda., and two groups of piezoelectric elements 4 are arranged circumferentially around thereof and secured to one surface of the vibration member 3 to provide a stator 2. These piezoelectric elements 4, in each group, are arranged at a pitch of .lambda./2 and so as to be of expansion-contraction polarity and are also arranged in such a manner that there is a deviation an odd number times as great as .lambda./4 between the two groups. Electrode films are provided on the two groups of piezoelectric elements. If an AC voltage is applied only to one group (hereinafter referred to as the A phase), a standing wave (wavelength .lambda. ) of such flexural vibration that the middle point of each piezoelectric element of the A phase and a point spaced, apart by .lambda./2 therefrom are the positions of antinodes. The middle point between the positions of the antinodes is the position where a node is produced in the vibration member 3 over the full periphery thereof. If an AC voltage is applied only to the other group (hereinafter referred to as the phase B), a standing wave is produced in a similar manner, but the positions of the antinodes and node deviate by .lambda./4 relative to the standing wave. If AC voltages identical in frequency and having a time phase difference .lambda./4 therebetween are applied to the A and B phases at the same time, the standing waves of the two are combined together with a result that a travelling wave (wavelength .lambda.) of flexural vibration travelling in the circumferential direction is produced in the vibration member 3. At this time, each point on the other surface of the vibration member 3 having a thickness effects a kind of elliptical movement. Consequently, if a ring-like moving member as a rotor 1 is brought into pressure contact with said other surface of the vibration member 3, the moving member 1 is subjected to a circumferential frictional force from the vibration member 3 and is rotatively driven. The direction of rotation of the moving member can be reversed by changing over the phase difference between the AC voltages applied to the A phase electrodes 5a and the B phase electrodes 5b of the two A and B phases to the positive or the negative. What has been described above is the epitome of the principle of operation vibration wave motor of this type.
In the vibration wave motor thus constructed, there are two types of control of the number of revolutions of the vibration wave motor, i.e., the system of varying the applied frequencies of the driving AC signals applied to the A phase and the B phase, and the system of varying the voltages of the driving AC signals applied to the A phase and the B phase.
According to the former system of controlling the number of revolutions, as shown in FIG. 5 of the accompanying drawings, there is an abrupt variation in the number of revolutions on the side of the frequency lower than the mechanical resonance frequency of the vibration wave motor, and this leads to numerous problems in stably driving the motor over a wider range of the number of revolutions. The latter system of controlling the number of revolutions has no abrupt point of variation in the number of revolutions as in the former system and therefore, it is a control method suitable for a case where the system is utilized over a wide range of the number of revolutions, and in this case, it is popular that the frequency of the driving AC signal applied to the vibration wave motor is fixed at a mechanical resonance frequency which is highest in driving efficiency.
Now, the mechanical resonance frequency of the vibration wave motor varies for each individual motor and with the temperature during driving. Therefore, the mechanical resonance frequency is not always provided at a fixed frequency.
So, in the vibration wave motor of this type, there has been provided a driving circuit of the PLL control system in which, besides the piezoelectric elements of the A phase and the B phase, a vibration detecting piezoelectric element (hereinafter referred to as the S phase) is secured to the vibration plate. The frequency of the AC voltage applied to the A phase and the B phase is automatically made into a resonance frequency in conformity with the detection output from the S phase electrode 5s of the S phase, whereby the vibration wave motor can be driven most efficiently.
However, in the driving circuit for the vibration wave motor by such PLL control system, as shown in FIG. 4 of the accompanying drawings, the output of the S phase is indefinite during the start of the motor or when the driving voltage is low has led to a case where the PLL control becomes impossible.